Nanotubes such as carbon nanotubes, boron nitride nanotubes and silicon nitride nanotubes, which possess a unique and microfine structure, are attracting much attention as a material capable of exhibiting an unprecedented new function.
While nanotubes include a monolayer nanotube with its outer wall made of one layer and a multilayer nanotube with its outer wall made of a plurality of layers, monolayer nanotubes have been found to exhibit an especially excellent function. For example, carbon nanotubes which have negative electron affinity exhibit excellent properties as an electron emissive material that emits electrons by field emission. Since their efficiency of electron emission depends on the curvature of their tips, nanotubes which are monolayer excel over multilayer nanotubes which are larger in diameter. It has become evident that a cold cathode tube using monolayer nanotubes has an extremely low threshold voltage and a light emitting device using such a cold cathode tube has an emission quantum efficiency that exceeds that of a GaN light emitting diode which presently is said to be extraordinarily high.
Also, in order to raise the mechanical strength of a metal or a synthetic resin, it has so far been customary to compound it with a fiber reinforcement material into a composite material. For example, use has been made of glass fibers and carbon fibers and more recently of multilayer carbon nanotubes as the material reinforcement. The smaller the fibers or tubes in size and the greater the uniformity of their distribution, it is known that the greater is the mechanical strength increased. If monolayer carbon nanotubes which are smaller in diameter are replaced for the multilayer carbon nanotubes, there should then ensue a further increase in the mechanical strength.
Also, while hydrogen is expected to be a major fuel in the future, it is difficult to store it safely and at high density. A method has so far been proposed using a hydrogen storage alloy of nickel family but this method has failed to be put to practical use because of the weight of nickel and its low ratio of weight to energy density. On the other hand, while monolayer nanotubes which are hydrogen absorptive and extremely low in weight are expected to be an excellent hydrogen storage material, this hydrogen storage material has not been put to practical use either because of extreme difficulties which have so far been encountered to mass-produce monolayer nanotubes.
Further, while metal containing fullerene is a functional nano-material that is similar in structure to nanotubes, it, too, has failed to be put to practical use because of the difficulty of producing fullerene of a particular structure at a satisfactory yield. Metal containing fullerene is useful as a catalyst in various technical fields due to its unique structure. The type of metal included in the fullerene will determine the situations in which it may be utilized as a catalyst.
In order to form monolayer carbon nanotubes, a method has been proposed in which a multilayer carbon nanotube produced is heat-treated in a plasma of oxygen, carbon dioxide or hydrogen to gasify multi-wall graphite layers of the multilayer carbon nanotube, thus forming a single-walled carbon nanotube. See J. Mater. Sci. 34, 1169 (1999). In this method, however, the graphite layers are gasified through an exothermic reaction whose rate of reaction changes rapidly, making it difficult to control the number of the layers to be gasified, and the gasification does not proceed uniformly over the entire product. Thus, the method has presented the problem that its yield is extremely low.
A method has also been proposed in which an oxidizing agent such as KMnO4, H2O2, O3 or HClO4 is used to gasify the multi-walled graphite layers of a multilayer carbon nanotube, forming a single-walled or monolayer carbon nanotube. See Solid State Ionics 141-142, 203 (2001).
In this method of using an oxidizing agent, however, the structure of a carbon nanotube is liable to change and it is hard to form a monolayer carbon nanotube of perfect structure at an acceptable yield.
Thus, while monolayer nanotubes or metal containing fullerene are hoped for as new materials capable of exhibiting various new functions, there has so far been made no method available that can be used to make them at low cost, namely at an industrially acceptable yield.